The invention relates to chemical processes used in the distillation of industrial chemicals. More particularly, the invention is directed to the recovery of acetic acid used during the production of terephthalic acid.
Terephthalic acid is useful in a diverse variety of industrial applications and chemical processes. For example, terephthalic acid is starting material for producing polyesters including plastic and Dacron™ polyester used in textile and container production. Polyethylene terephthalate (PET) is a form of polyester or Mylar™ that is an extremely tough resin and useful in many industrial and consumer applications. Soft drink and water bottles are made from this resin in addition to plastic jars and clamshell packages used in consumer good transport and food distribution.
Terephthalic acid is typically produced by a reaction of paraxylene with molecular oxygen in the presence of catalysts. During the production process, acetic acid is used as a solvent of terephthalic acid. The acetic acid becomes diluted in water during the oxidation in a reactor section of a terephthalic acid plant in the production cycle. A portion of the acetic acid and water stream is then sent to a dehydration unit to remove the water generated in the reactor for recycling or waste.
Three different approaches have been employed in the terephthalic acid plants to separate the acetic acid and water so that the acetic acid can be recycled back to the reactor while the water generated by the reaction is sent to the waste water treatment facility for safe processing. One approach is by convention distillation wherein the different boiling point of the components provide for the separation of acetic acid and water. An azeotropic distillation approach utilizes entrainers to form azeotropes with the acetic acid and water providing for a change in energy requirements for processing. Liquid-liquid extraction is a final approach for acetic acid and water separation during the terephthalic acid production.
Distillation has been widely used as a primary unit operation for acetic acid recovery from water. In such processes, one or more towers are utilized to process a number of streams of varying concentrations of acetic acid with the purpose of recovering it for further use in an oxidation step. The products from the distillation tower are a bottom stream of concentrated acetic acid and an overhead stream that ideally would be pure water to minimize the loss of the valuable acetic acid solvent. A more pure overhead water stream would also reduce the burden on downstream waste water treatment facilities thereby preventing accidental chemical spills.
However, the distillation of acetic acid and water is not very efficient due to the highly non-ideal vapor/liquid equilibrium characteristics of the acetic acid/water system. Conventional distillation systems require the use of high number of theoretical stages, i.e., actual trays, and a high reflux ratio, i.e., high energy consumption, to obtain reasonably low levels of acetic acid, typically in the range of 0.5–0.8 wt % in the overhead distilled water. The overhead waste water is subsequently processed to recover certain organic by-products, and then, sent to the waster water treatment facility where any remaining acetic acid will be neutralized and spent.
The use of conventional distillation, therefore, involves high investment cost because of the large dimensions of the required tower and column equipment and a high operating cost because of the high amounts of steam consumption involved. Furthermore, the traditional process scheme does not allow one to economically obtain a distillate completely free of acetic acid. This limitation, in turn, presents operating problems including costs associated with the operation resulting from the acetic acid losses, costs associated with the treatment of the acetic acid in the waste water, limitations of the capacity of the downstream waste water treating facility and environmental problems that are continually increasing because of the ever more rigorous standards for acceptable levels of emission to the environment.
There has been an effort to look for alternatives to minimize the high operating costs associated with the conventional distillation for the separation of acetic acid and water. Chemical processors and companies have resorted to azeotropic distillation involving the addition of selective alkyl acetate, such as the isobutyl acetate (IBA), normal butyl acetate (NBA), normal propyl acetate (NPA) etc., as an entrainer to the dehydration column. The entrainer forms a low boiling azeotrope with water and therefore improves the relative volatility of the separation between the acetic acid and the alkyl-acetate/water azeotrope. This reduces the energy or theoretical stage requirements for the same separation. Compared to the conventional distillation, an azeotropic distillation approach typically reduces the energy (i.e., steam) consumption by 20–40% to the acetic acid/water dehydration column while giving relatively low acetic acid concentration, 300–800 ppm, in the distillated water. The azeotropic distillation column is generally operated at ambient pressure in the terephthalic acid plants in all prior art systems.
Effort has also been reported by the use of liquid-liquid extraction with special extractive agents to recover the acetic acid from the water streams to only contain 0.1 wt % acetic acid to 20% acetic acid. Some of the agents usually used are acetates, amines, ketones and phosphine oxides and mixtures thereof. Once the extraction step is completed, a complicated series of distillation steps are required to recover the acid and to recirculate the extractive agent back to the extraction stage.
In view of such energy waste from the use of traditional distillation systems, what is needed is a distillation system which is energy efficient and produces less waste and unwanted byproducts. The system should also recycle both energy and initial products in a environmentally friendly manner. The distillation system and process should also be easily modifiable to existing chemical process systems to enhance current and existing plants. Moreover, the recovery system should be easy to install without large capital expenditures.
For these reasons, it would be desirable for a distillation system for recovering acetic acid to use less energy, generate energy for other uses within the plant. The system should save raw chemicals and material, result in less acetic acid runoff in waste water and save money. Such systems and methods should also be applicable in a wide variety of chemical processes on a wide range of industrial chemicals. The system should also be simple and less expensive to manufacture while being compatible with conventional systems and processes.